1. Field of the Invention
The invention relates to a multi-layer optical planar waveguide which is vertically coupled using multimode interference (MMI) couplers and to the method of manufacturing the same. The purpose of this invention is to increase the degree of integration on the multi-layer optical planar waveguide by applying the concept of via holes of the multi-layer printed circuit board (MLPCB) used in electronic circuits to the optical waveguide devices.
2. Description of the Prior Art
As the technology of optical communications are advanced, various optical planar waveguide technologies have been developed in order to fabricate various integrated optical components such as optical couplers optical switches, optical amplifiers, and so on.
Recently, as the optical planar waveguide has been actively applied in optical communications, the higher integration of the optical planar waveguide devices and the integration of various optical planar waveguide devices on an optical board are frequently demanded. For example, U.S. Pat. No. 6,115,515 (hereinafter, xe2x80x98reference document 1xe2x80x99) entitled xe2x80x9cOptical Device Mounting Boardxe2x80x9d issued to M. Itoh, et al. on Sep. 5, 2000 discloses a method by which various optical planar waveguide devices are transversely arranged on an optical board and connected respectively.
The reference document 1 discloses the invention allowing simple attachment of photoelectric devices such as the optical waveguide device as indicated in the title. In the reference document 1, various optical planar waveguide devices are arranged on a mounting board on which grooves are engraved, so that the waveguides are easily connected. This technical idea is one of the methods to connect the optical planar waveguide devices liven though this technical idea can partly contribute to overcoming the difficulty in the connection of various optical waveguide devices, the process should accompany the economical problem in using the expensive aligning equipments, the problem of long process time and the complexity in fabrication. Also, as in the reference document 1 there is a limit in integration using the technical idea in which the optical planar waveguides are transversely connected on a board.
The multi-layer optical planar waveguide with the vertical structure can be another way to simplify the integration of waveguide devices. 1, or example, the multi-layer optical planar waveguide of the vertical structure was disclosed in xe2x80x9cDouble-Bonded InPxe2x80x94InGaAsP Vertically Coupler 1:8 Beam Splitterxe2x80x9d (hereinafter, xe2x80x98reference document 2xe2x80x99) by M. Raburn, et al in IEEE Photon. Technol. Lett. (2000) and xe2x80x9cInPxe2x80x94InGaAsP Wafer-Bonded Vertically Coupled X-Crossing Multiple Channel Optical Add-Drop Multiplexerxe2x80x9d (hereinafter, xe2x80x98reference document 3xe2x80x99) by M. Raburn, et al in IEEE Photon. Technol. Lett. (2001).
The reference document 2 discloses a 1:8 beam splitter in which optical planar waveguides of multiple layers are formed in order to increase the degree of integration. The reference document 2 tried to improve the degree of integration of the optical planar waveguide device, by using the coupling characteristics of evanescent field interference between the upper and the lower optical planar waveguides, in which the gap between the upper and the lower layers is narrower than 1 xcexcW, or narrower than the width of the waveguide itself. In addition, the reference document 3 is about a filter using optical waveguides of two layers and discloses a technical idea in which the degree of integration of the optical planar waveguide device is improved using the coupling characteristics of evanescent field interference between the upper and the lower waveguides as in the reference document 2. These prior arts, however, have a significant limitation that the optical planar waveguide on the upper and the lower layers is highly dependent each other since the gap between the upper and the lower layers is too narrow. There should be high interference between the layers due to the evanescent field with such a narrow gap.
In addition, technologies about the multi-layer optical planar waveguide with vertical structure include xe2x80x9cVertically Stacked Coupler and Serially Grafted Waveguide: Hybrid Waveguide Structures Formed Using an Electro-Optic Polymerxe2x80x9d (hereinafter, xe2x80x98reference document 4xe2x80x99) by T. Watanabe, et al in xe2x80x98J. Appl. Phys. (1998), and U.S. Pat. No. 6, 282, 335 (hereinafter, xe2x80x98reference document 5xe2x80x99) entitled xe2x80x9cThermo-Optic Switchxe2x80x9d (Aug. 28, 2001).
The reference document 4 discloses an electro-optic switch with relatively low loss using two layer optical waveguides with the lower layer made of electro-optic material, whose loss is generally quite high, and the upper layer made of passive material with low loss. In addition, the reference document 5 discloses a thermo-optical switch in which the main waveguide is formed with glass and in which polymer waveguide with high thermo-optical coefficient is transversely arranged on the glass waveguide, so that light travels from the glass waveguide to the polymer waveguide controlled by the heat applied to the polymer. These two reference documents 4 and 5, however, have the same limitation that the optical planar waveguide on the upper and the lower layers is highly dependent each other since the gap between the upper and the lower layers is too narrow. There should be high interference between the layers due to the evanescent field with such a narrow gap.
As described above, the documents referred made some progress in implementing a switch or a filter using the multi-layer optical planar waveguide structure but have a significant limit in designing because the layers cannot be operated independently in case they are overlapped while the gap between the upper and lower layers should be maintained as narrow as possible to couple the layers directly by the evanescent field interference.
Meanwhile, a technical idea of implementing a multi-layer optical planar waveguide of an independent structure includes xe2x80x9cMultilevel Registered Polymeric Mach-Zehnder Intensity Modulator Arrayxe2x80x9d (hereinafter, xe2x80x98reference document 6xe2x80x99) by T. A. Tumolillo, Jr. et al in Appl. Phys. Lettxe2x80x9d (1993). In the reference document 6, a multi-layer optical planar waveguide device with little interference between the upper and lower layers is implemented. However, the reference document 6 does not disclose how to connect the upper and lower layers. Therefore, there is a need for a technology by which the optical waveguide on the upper and the lower layers can be vertically connected while the interference between the each layer of the multi-layer optical planar waveguide is minimized.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a multimode interference coupler capable of connecting the optical waveguides on the upper and the lower layers with high coupling ratio at a specialized spot while maintaining the interference between the each layer of the multi-layer optical planar waveguide out of the spot as minimum by keeping the each layer at an enough distance.
Another object of the present invention is to provide a multimode interference coupler with at least one stepped structure in order to increase the coupling ratio between the optical waveguides on the upper and the lower layers.
Still another object of the present invention is to provide a multi-layer optical planar waveguide implemented using a multimode interference coupler of the stepped structure.
Still another object of the present invention is to provide a method of manufacturing a multi-layer optical planar waveguide with a multimode interference coupler.
In order to accomplish the above object, a multimode interference coupler according to the present invention, is characterized in that it comprises an optical waveguide for a multimode interference coupler inserted at a specialized region between a lower optical waveguide and an upper optical waveguide so that the lower optical waveguide and the upper optical waveguide can be vertically connected, wherein the thickness of the optical waveguide for the multimode interference coupler is larger than the thickness of the optical waveguide to minimize the interference between the lower and the upper optical waveguide out of the region.
Further, a method of manufacturing a multi-layer optical planar waveguide according to the present invention, is characterized in that it comprises the steps of forming a lower core layer at a given portion of a substrate; coating a clading material on the entire structure; etching the clading material so that a portion of a lower core layer at a region where a multimode interference coupler will be formed, thus forming a lower clad layer; coating a core material on the entire structure; etching the core material to form a multimode interference coupler and an upper core layer; and coating the cladding material on the entire structure to form an upper clad layer.
In addition, a method of manufacturing a multi-layer optical planar waveguide according to the present invention, is characterized in that it comprises the steps of forming a lower core layer at a portion of a glass substrate using ion-exchanged method; coating photosensitive polymer the refractive index of which is changed by means of UV light on the entire structure; forming an upper core layer at a portion in the photosensitive polymer; forming an upper clad layer on the entire structure; locating a metal mask on the upper clad layer corresponding to a region where a multimode interference coupler will be formed; and illuminating an UV light on the entire structure to form the multimode interference coupler and a lower clad layer in the photosensitive polymer.